A structural material for transportation equipment such as automobiles, railroad vehicles, and aircrafts is required to have performance such as (1) strength, (2) corrosion resistance, and (3) fracture mechanics properties (such as fatigue crack propagation resistance and fracture toughness). A recent material development trend involves overall evaluation including not only strength, but also production, assembly, and operation of the material.
As high-strength aluminum alloys, an Al—Cu—Mg aluminum alloy (2000 series) and an Al—Zn—Mg—Cu aluminum alloy (7000 series) have been known. These aluminum alloys exhibit excellent strength. However, these aluminum alloys do not necessarily exhibit sufficient corrosion resistance, and tend to produce cracks due to inferior extrudability. Therefore, since these aluminum alloys must be extruded at a low extrusion rate, manufacturing cost is increased. Moreover, it is difficult to extrude these aluminum alloys into a hollow product by using a porthole die or a spider die. Therefore, since it is necessary to form a desired structure by combining solid profiles, the application range of these aluminum alloys is limited.
A 6000 series (Al—Mg—Si) aluminum alloy, represented by an alloy 6061 and an alloy 6063, allows easy manufacture due to excellent workability, and exhibits excellent corrosion resistance. However, the 6000 series alloy exhibits insufficient strength in comparison with the 7000 series (Al—Zn—Mg) or 2000 series (Al—Cu) high-strength aluminum alloy. An alloy 6013, alloy 6056, alloy 6082, and the like have been developed as the 6000 series aluminum alloys provided with improved strength. However, these alloys do not necessarily exhibit strength and corrosion resistance sufficient to meet a demand for a reduction in the material thickness along with a reduction in the weight of vehicles.
In order to solve the above-described problems relating to the 6000 series aluminum alloys to obtain a high-strength aluminum alloy extruded product exhibiting excellent corrosion resistance, JP-A-10-306338 proposes an Al—Cu—Mg—Si alloy hollow extruded product containing 0.5 to 1.5% of Si, 0.9 to 1.6% of Mg, 1.2 to 2.5% of Cu while satisfying conditional expressions “3%≦Si %+Mg %+Cu %≦4%”, “Mg %≦1.7×Si %”, “Mg %+Si %≦2.7%”, “2%≦Si %+Cu %≦3.5%”, and “Cu %/2≦Mg %≦(Cu %/2)+0.6%”, and further containing 0.02 to 0.4% of Cr and 0.05% or less of Mn as an impurity, with the balance being aluminum and unavoidable impurities, in which, when a tensile test is conducted for a weld joint inside a hollow cross section formed by extrusion in the direction perpendicular to the extrusion direction, the aluminum alloy extruded product breaks at a position other than the weld joint.
As an aluminum alloy extruded product of which the strength is improved by adding Mn to the above aluminum alloy extruded product and in which the corrosion resistance is maintained by controlling the thickness of the recrystallization layer of the extruded product, JP-A-2001-11559 proposes an aluminum alloy extruded product containing 0.5 to 1.5% of Si, 0.9 to 1.6% of Mn, 0.8 to 2.5% of Cu while satisfying conditional expressions “3%≦Si %+Mg %+Cu %≦4%”, “Mg %≦1.7×Si %, Mg %+Si %≦2.7%”, and “Cu %/2≦Mg %≦(Cu %/2)+0.6%”, and containing 0.5 to 1.2% of Mn, with the balance being aluminum and unavoidable impurities, in which, when the minimum thickness of the extruded product is t (mm) and the extrusion ratio is R, the thickness G (μm) of the recrystallization layer on the surface of the extruded product satisfies “G≦0.326 t×R”.
In the above aluminum alloy extruded product, the microstructure other than the recrystallization layer in the surface layer is made fibrous by adding Mn. Although the strength of this aluminum alloy extruded product is improved by this measure, a problem relating to extrudability, such as extrusion cracks, occurs depending on the conditions. Therefore, one of the inventors of the present invention, together with another inventor, proposed a method of improving extrudability by, when extruding a solid product by using a solid die, extruding a solid product under conditions where the bearing length of the solid die and the relationship between the bearing length and the thickness of the extruded product are specified, and, when extruding a hollow product by using a porthole die or a bridge die, extruding a hollow product under conditions where the ratio of the flow speed of the aluminum alloy in a non-joining section to the flow speed of the aluminum alloy in a joining section, in which the billet rejoins after entering a port section of the die in divided flows and subsequently encircling a mandrel, is controlled (JP-A-2002-319453).
These extruded products are generally used after being subjected to secondary working such as bending or machining after extrusion (primary working). However, since the above aluminum alloy extruded product containing Mn has a recrystallized structure in the surface layer and a fibrous structure inside the product, the surface properties and the dimensional accuracy after secondary working are decreased if the recrystallization texture becomes coarse. As a result, a severe dimensional tolerance may not be maintained or machinability may be decreased.